Variable amplitude equalizer circuits are used in many transmission systems and particularly in long distance telephone circuits to compensate for the ambient operating conditions of the system. U.S. Pat. No. 2,096,027 by Hendrik W. Bode, issued Oct. 19, 1937, described an adjustable attenuation equalizer which is the basis for many such circuits used today.
In the implementation of many of these variable amplitude circuits it is often necessary to vary two circuit components (usually resistors) simultaneously in order to avoid interaction between adjustable parameters. When a discrete step-adjustment technique is required, a chain of resistors and multi-pole switches are used to perform this function. If, however, a continuous adjustment is required, dual ganged potentiometers are used. Either of these techniques requires the use of expensive components which aside from being bulky have inherent tracking problems.
U.S. Pat. No. 3,921,105 issued Nov. 18, 1975 to Franc Brglez discloses a variable attenuation equalizer in which a single variable impedance element is used to provide variable magnitude equalization.